Yeast fermented beverages, such as beer, are stabilised to ensure that the beverage tastes and looks as good at the end of its shelf life as it did after packaging. Since a consumer's first evaluation is a visual one, clarity is taken as a determining measure of beer quality. With a few notable exceptions, consumers expect a bright, appealing product, which is free from haze.
Colloidal haze in beer arises from the formation of polyphenol-protein complexes during storage. Fresh beer contains acidic proteins and a variety of polyphenols. Whilst these can form complexes via loose hydrogen bonding, their low molecular weight means that they are too small to be visible to the naked eye. As these small polyphenols, termed flavanoids, polymerise and oxidise, they produce short chain (condensed) polyphenols termed tannoids. These tannoids are capable of bridging across a number of proteins through hydrogen bonding to form reversible chill haze. After further storage, stronger ionic and covalent bonds form between the tannoids and proteins resulting in irreversible permanent haze. The rate and extent to which this occurs is impacted by the brewing materials, process and storage conditions and can be greatly improved (reduced) by the use of stabilisation aids.
Since the rate-determining factor in the development of haze is the change in the polyphenol fraction, reducing the levels of these haze precursors is a very efficient method of ensuring the colloidal stability of beer. Polyvinylpolypyrrolidone (PVPP) is a cross-linked polymer of (poly)vinylpyrrolidone that is insoluble in water. Highly porous PVPP particles are used in the brewing industry for adsorption of haze polyphenols. PVPP selectively complexes haze polyphenols, predominantly through very strong hydrogen bonding, with multiple attachment sides for haze polyphenols. The molecular structure of the PVPP polymer limits internal hydrogen bonding, maximising the number of available reactive sites.
PVPP stabilisers are either optimised for single use, where they are added to the beer stream and removed on the kieselguhr filter or, for regeneration grades, added to bright beer using dedicated filtration units and recycled for reuse. In either mode many of the initial handling characteristics are common. The PVPP powder is slurried in the dosing tank using softened de-aerated water at a concentration of around 8-12% (wt./vol.). The material must be stirred for at least 15 minutes to swell and hydrate the particles. The slurry should then be kept under constant agitation to prevent settling. In the case of regeneration grades, the stabiliser dosing tank is often maintained at 80° C. to ensure long-term microbial stability.
The most common method of adding single use PVPP is by continuous dosing to the beer stream using a proportioning pump. Although PVPP can be very effective with short contact times, a contact time of 5-10 minutes between the point of addition and removal of the spent PVPP on the kieselguhr filter is recommended for maximum efficiency. PVPP should be added to cold beer, at or below 0° C., to prevent re-dissolution of those polyphenol-protein complexes that have already formed.
The principle of regenerable PVPP use is to break the PVPP-polyphenol bonds through washing the material with a caustic (NaOH) solution. Regeneration is considered to be economic if a brewery stabilises a large output volume and/or the beer being stabilised has an extremely high polyphenol content, that would require high addition rates of PVPP for effective colloidal stabilisation. Regeneration grades of PVPP are specifically manufactured to produce particles of larger size and greater mechanical strength, which still give effective polyphenol reduction. Horizontal leaf filters were the original designs for using and regenerating PVPP, but candle filters are also now entering service.
The initial preparation of regeneration grades of PVPP is very similar to that of the single use product. A dedicated slurry tank is required, often equipped with a heating jacket. The empty filter is first purged with CO2 and a pre-coat of regenerable PVPP of about 1-2 mm in depth is deposited on the filter screens of. The stabiliser slurry is recirculated around the filter till the water at the sight glass or measuring point is clear. PVPP is dosed into the now incoming beer stream using a proportioning pump. The effective stabilisation run is completed when the space between the filter plates is filled with PVPP. The final volume of beer stabilised depends on the size of the filter, PVPP charge and the addition rate into beer and can run to several thousand hl.
At the end of the filtration and stabilisation, residual beer is returned to the beer recovery tank. The used PVPP is regenerated by circulating a caustic (1-2% wt/wt) solution, at 60-80° C. through the PVPP filter bed for between 15-30 minutes. Sometimes, a second caustic rinse is used, with the first cycle run to drain and the second cycle saved for re-use as the first caustic rinse at the next regeneration. The colour of the caustic leaving the filter is very dark, confirming the breakage of the strong PVPP-polyphenol complexes. The PVPP filter cake is then flushed with hot water at 80° C. to displace the caustic solution and reduce pH. This is followed by a rinse cycle with dilute acid until the solution leaving the filter reaches around pH 4 over 20 minutes. Residues from the beer and water are effectively removed and best results are achieved by pre-heating the dilute acid to around 60° C. The filter is then flushed with cold water until the acid is washed out and the pH at the outlet is neutral. Finally CO2, water and the centrifugal force of spinning the filter elements are used to displace the regenerated PVPP from the filter screens to the dosing pot. The solids (PVPP) content in the dosing tank is checked and new material added to make up for process losses. These losses are typically between 0.5-1% per regeneration. However, it is the cost of the filter hardware, rather than that of the PVPP stabiliser, that has a more significant influence on the economics of PVPP regeneration.
Thus, whereas single use PVPP has the disadvantage that it generates a considerable waste stream, regenerable PVPP suffers from the drawback that it requires a considerable upfront investment in sophisticated filter hardware.
WO 99/16531 describes a process for the regeneration of spent filter media that has been used in mechanical filtration of beer and that contains perlite and PVPP. The regeneration process disclosed in WO 99/16531 comprises the following steps:    adding an aqueous liquid comprising about 0.25 to 3.0 weight percent caustic to a regeneration vessel containing a filter cake comprising a filter media and filtrates;    stirring the contents of the regeneration tank for a time not exceeding 18 hours at a temperature not exceeding about 110° F. (43.3° C.);    substantially removing the aqueous liquid from the filter media;    rinsing the filter media with a caustic solution;    rising the filter media with an acid solution; and    rinsing the filter media with water.Effective removal of yeast cells in this process relies on caustic degradation or modification of these yeast cells and the removal of the degraded/modified yeast cells during rinsing operations.